'Cruel and unusual'... Dennis McGuire was executed by means of a two-drug lethal injection process never before tried in the US. Picture: Ohio Department of Rehabilitation and Correction Source: AP

THE family of an Ohio killer who suffered a prolonged execution plans to sue the state over its use of a lethal injection process never before tried in the US.

"I can't think of any other way to describe it than torture," Amber McGuire, the adult daughter of Dennis McGuire, who was put to death on Thursday, told NBC.

Amber McGuire and her brother watched on as their 53-year-old father made loud snorting noises during an execution that lasted nearly 25 minutes.

Dennis McGuire's lawyers had attempted to delay his execution by arguing that the untested drug method could lead to a medical phenomenon known as "air hunger" which could cause him to suffer "agony and terror."

While the court acknowledged the execution would be an “experiment," it ultimately sided with the state and cleared the use of the deathly dose of two new chemicals.

McGuire’s family plans to file a lawsuit against the Ohio Department of Rehabilitation and Corrections "to make sure that this procedure is not utilised on anyone else ever," the family’s lawyer, Jon Paul Rion, told NBC.

Mr Rion said the procedure, adopted after the maker of the state's previous drug put it off limits for capital punishment, violated America’s Eighth Amendment, which prohibits the use of cruel and unusual punishments.

McGuire, who was sentenced to death for the 1989 rape and murder of a young pregnant woman, was killed using a combination of midazolam, a sedative, and the painkiller hydromorphone.

Mr Rion said McGuire "spoke to his son and made his son promise that if things don’t go right -- because everyone feared that it wouldn’t -- he would pursue this to make sure that it wouldn’t happen to anyone else."

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